Continuous time cross-correlator

ABSTRACT

There is provided a continuous time cross-correlator comprising: a quantizer for quantizing the incoming signal into discrete levels; a delay line comprising one or more delay units separating a plurality of delay line taps; for each of said delay line taps, a comparator for comparing the signal level of the delay line tap with a correlation value; a continuous time counter for taking the outputs of the plurality of comparators as its inputs, counting the results of the comparisons and outputting the results of the comparisons; and an output comparator for comparing the counter output with a threshold value. The cross-correlator provides for high speed continuous time cross-correlation with low power consumption and a small chip area. Methods of continuous time cross correlation are also provided.

The invention relates generally to cross-correlators and morespecifically to a pulse detector or symbol detector which operates incontinuous time.

In Direct Sequence Spread Spectrum (DSSS) or Code Division MultipleAccess (CDMA) systems, data bits are represented by various symbols (orcodes), each symbol being a sequence of chips. The chip frequency (chiprate) is therefore higher than the symbol frequency (baud rate). Chipsequences can either by pseudorandom (pseudonoise) sequences or they canbe specifically chosen codes (e.g. Walsh codes, Hadamard codes, Graycodes, Barker codes). By using orthogonal codes, different codes can beused to distinguish different bit sequences and/or differenttransmission sources (e.g. different users). By varying the length ofthe codes, the number of available codes can be varied.

It is vital for receivers to be able to detect incoming symbolsreliably. This can be a difficult task. Current symbol detectors requireaccurate clock synchronisation with the transmitter in order todetermine the time windows in which to look for an incoming symbol.Accurate clock synchronisation also ensures that the chip rate of thesender and receiver are closely matched which is important for accuratesymbol matching. The high frequency clocks used in these symboldetectors require relatively high power which puts limitations on thesize of the receivers and limits the time that such devices can operateon battery power. As this technology is central to many wireless and/ormobile communication systems, size and power consumption are importantconcerns.

In existing technology, a typical symbol detector takes an incominganalog signal and samples it at a high rate in order to digitize itusing an Analog to Digital Converter (ADC). Subsequent processing of thedigital signal is done in Digital Signal Processors (DSPs) which canstore the digitized signal in memory and subsequently manipulate and/oranalyse it. The DSPs require high power and also occupy significantamounts of chip area. The same applies to the clocks required forsampling and processing.

According to a first aspect, the invention provides a continuous timecross-correlator comprising: a quantizer for quantizing the incomingsignal into discrete levels; a delay line comprising one or more delayunits separating a plurality of delay line taps; for each of said delayline taps, a comparator for comparing the signal level of the delay linetap with a correlation value; a continuous time counter for taking theoutputs of the plurality of comparators as its inputs, counting theresults of the comparisons and outputting the results of thecomparisons; and an output comparator for comparing the counter outputwith a threshold value.

This continuous time-domain counter provides a power-efficientstatistical receiver which is viable for a high-speed symbol detectorsuitable for use in impulse radio receivers and also forpulse-detection, particularly of higher-order Gaussian pulses. Theinvention addresses the computational challenge of detecting noise-like(or periodic) sequences of pulses in the time domain usingcross-correlation with an expected template. The invention operates incontinuous time and is less computationally demanding than existingreceivers which use a predominantly digital approach and therefore havesignificantly higher power consumption. The receiver proposed here hasthe further advantage of not requiring a high speed clock. Suchasynchronous (clockless) operation provides another significant powersaving.

By retaining the received signal in continuous time throughout theprocessing, rather than sampling it into discrete time, the circuitavoids the need for complex digital signal processing architecture andfurther avoids the need for high speed clocks. Instead of sampling thesignal, the signal is passed through a delay line and multiple delayedversions of the signal are tapped off the delay line at regularintervals so that different time-domain parts of the signal areprocessed simultaneously. The delayed signals are passed into acontinuous time counter which continuously compares each of the delayedsignals with a given correlation bit. The counter thus outputs acontinuous time representation of the correlation between the incomingsignal and the correlation values. The delay line is an asynchronous(non-clocked) delay line.

The correlation values may be an array of values making up a bitsequence corresponding to a symbol template or to another signal(cross-correlation) or to the same incoming signal (auto-correlation).The correlation values may be static in time (as in a symbol template)or time variable (as in cross-correlation and auto-correlation).

Providing that the delays between the delay line taps match the chiprate of the transmitted bit sequence, the inputs to the counter willrepresent time-domain samples spaced apart by the chip rate. Thereforein the case of symbol detection for example, as a symbol in the receivedsignal passes through the delay line, all of the counter inputs shouldmatch the template bits at the same time leading to a high count at theoutput of the counter representing high correlation.

The use of a continuous time counter avoids the need for digitalelectronics and allows the processing to be carried out at high speedand high power efficiency. Further, the counter output can easily becompared against a statistical threshold so that the level of templatematching required for symbol detection can be varied. The thresholdvalue for comparison may be a static value or it could be time-variable.The threshold value may incorporate a time element to take account ofsignal history. For example it could require the count to remain above acertain value for a certain period of time in order to detectcorrelation.

Preferably the counter comprises: a plurality of counter inputs; and aplurality of adding stages, each adding stage comprising a stack of oneor more outputs, and each counter input providing an input to an addingstage, wherein the adding stages are arranged successively from a firstadding stage to a last adding stage, wherein the first adding stage isarranged to set its single output to a first output value, and whereineach succeeding adding stage is arranged to take the outputs from theimmediately preceding stage and if the appropriate counter inputrepresents a count, the adding stage is arranged to add a new output atthe bottom of the stack and if the counter input represents no count,the adding stage is arranged to add a new output at the top of thestack.

It will be understood that the terms “count” and “no count” are intendedto represent arbitrary values such as “0” and “1”, “high” and “low”,“positive” and “negative”, “true” and “false”, etc. Systems can be setup in a variety of ways with different system attributes representingthe desired values. The general principle of the counter outlined hereapplies to all such arrangements. It will also be appreciated that theterms “top” and “bottom” for the stack are arbitrary and are simply usedto distinguish opposite ends and are not intended to be limiting onorientation.

Preferably the first adding stage has its input determined by the firstcounter input and if the first counter input represents a count, thefirst adding stage is arranged to set its single output to represent acount and wherein if the first counter input represents no count, thefirst adding stage is arranged to set its single output to represent nocount, and in each succeeding adding stage, if the counter inputrepresents a count, the adding stage is arranged to add a new output atthe bottom of the stack representing a count, and if the counter inputrepresents no count, the adding stage is arranged to add a new output atthe top of the stack representing no count.

This counter operates by building up a thermometer code in stages. Thethermometer code is made up of an array of values each taking one of twovalues, with the sets of values grouped together at each end of thearray. At each stage one counter input is taken into account and itsstate is added to the thermometer count before the count is passed on tothe next stage. In this way, the size of the thermometer grows as itpasses through the counter, regardless of the values within it. Thefirst stage of the thermometer therefore contains just a single valuewhich is set according to the state of the first counter input.Succeeding stages have to make choice between adding a new input at thetop of the stack or at the bottom of the stack of thermometer values. Toretain the thermometer code, like values are always added at the sameend. So, if the counter input represents a count (i.e. takes a valuethat indicates that the counter value is to increase) then a count valueis added at the bottom of the stack, and if the counter input representsno count (i.e. takes a value that indicates that the counter value is tostay the same) then a no count value is added at the top of the stack.The final stage of the counter has taken into account all of the inputsand thus forms the output of the counter as a whole.

The advantages of this arrangement are that it operates in continuoustime and can therefore produce high speed outputs without consuminglarge quantities of power.

In an alternative arrangement, a first value is injected into thecounter at the first stage (which holds just one value) and eachsubsequent stage adds further elements differing from the first value tothe counter output code, thus building up the code in the same fashionas before. At each stage, the new inputs are added to the top or bottomof the stack as before, but the values added to the bottom of the stacksdo not need to be the same as the first value. At each adding stage, thestack therefore contains only a single element at the first value. Theheight of that element within the stack depends upon the inputs and thusrepresents the level of correlation.

The newly added inputs at each stage of the counter may be takendirectly from the corresponding counter inputs, but preferably they areprovided directly from a separate source e.g. a rail in the circuit tiedto the appropriate value. The inputs to the first stage may be takendirectly from the counter input in most cases, but in higher frequencyoperations, this approach could cause problems with the instability ofthe inputs combined with a short decision time. Providing independentvoltages (e.g. rails or hard-wired voltages) for the inputs to at leastthe second and subsequent stages of the counter avoids this instabilityand also decouples these inputs from the preceding circuitry, thusdecreasing load.

Viewed from an alternative perspective, the thermometer coded countermay comprise: a plurality of counter inputs; and for each counter input,a group of one or more switching elements, each switching element in thegroup being controlled by said counter input and said one or moreswitching elements being arranged from a least significant switchingelement to a most significant switching element; wherein the groups aresuccessively arranged and wherein each group comprises a plurality ofinputs and a plurality of outputs, the number of inputs being onegreater than the number of outputs, and wherein the plurality of inputscomprises a least significant input held at a level indicative of acount and a most significant input held at a level indicative of nocount, with inputs of intervening significance being formed from theoutputs of the preceding group; wherein the switching elements of eachgroup are arranged such that when the counter input is indicative of acount, the least significant input is connected to the least significantoutput and the remaining inputs except the most significant input areconnected to outputs of greater significance, and when the counter inputis indicative of no count, the most significant input is connected tothe most significant output and the remaining inputs except the leastsignificant input are connected to outputs of lesser significance.

In each adding stage, the inputs and outputs may be connected by anytype of logic, e.g. static gates (rail referred) or transmission gates(signal referred) or single devices (e.g. transformers). Preferably eachadding stage comprises one or more switching elements for connecting theinputs to the outputs. The switching elements provide connectionsbetween the outputs of one stage and the inputs of the next and alsoprovide connections for inserting the newly added inputs at each stage.The switching elements may take a variety of forms such as transistorsor physical switches. In preferred embodiments the switching elementsare multiplexers each arranged to connect one of two neighbouring inputsto an output or demultiplexers each arranged to connect an input to oneof two neighbouring outputs. At each stage of the counter, the outputsmust either be passed along to a corresponding position in the new stageor redirected up one place if a new input is being entered at the bottomof the stack. Therefore each output can be passed to one of two adjacentinputs in the next stack. Looked at the other way round, each input canbe taken from one of two adjacent outputs in the previous stack (exceptfor newly added inputs).

In small scale or lower speed applications (where the input pulses aresufficiently long), the counter will be able to function correctly withthe inputs simply being taken directly from the delayed signal streams.However in large and/or fast applications, the counter is preferablyarranged to apply a delay to each of its inputs to compensate for thepropagation delay through the adding stages. In a large counter withshort input pulses, the propagation delay for the first counter signalmay exceed the input pulse length. As all of the stages operate inparallel according to their individual input streams, this could resultin false counts and missed symbols. The compensation delay may be addedas part of the counter circuitry or it may be added elsewhere in thedetector circuit, e.g. by incorporating it into the delays in the delayline. The compensation delay may be provided on separate delay elementsfor each stage. Alternatively, as the inherent delay associated with theswitching elements can be varied by design, the components of thecounter may be designed to have an appropriately compensated delay.

The delay units of the delay line may be set and calibrated within thefactory. However this is difficult due to process variations, especiallyfor longer delay units. Therefore preferably the delay units of thedelay line are tunable. Tunable delay elements allow the delay line tobe accurately calibrated to a given chip rate. If the detector is partof a device which is designed for operation at a single specific chiprate, then the delay units can be tuned and calibrated once at the startof the product's life. This saves further chip area by avoiding the needfor further calibration elements. However, it is preferred that thedelay line be tunable in the field, e.g. to recalibrate the device whenit drifts (e.g. due to temperature or humidity variations) or torecalibrate for a different chip rate.

Tunable delay elements are normally created from circuit elements whichhave a slow response time. These circuits can be tuned by varyingcertain components of the delay circuit to increase or decrease thedelay. However these circuits have an unreliable output and accuratedelay calibration is very difficult. Power consumption is also notefficient. Preferably therefore each delay unit comprises a plurality offirst delay elements and a plurality of first delay element taps betweenthe first delay elements, wherein the first delay element taps areinputs to a first multiplexer and wherein the output of the firstmultiplexer is selected from among the inputs according to a first tapselect input. In this way, the amount of delay provided by each delayunit can be tuned by selecting how many delay elements within the delayunit are involved in the path between the input and the output of thedelay unit. Further each delay unit is easily tuned simply by varyingthe tap select input. All of this circuitry can be made from simple,low-power circuit elements such as inverters and multiplexers. Further,as the shortest delay element can be made from two inverters in serieswith a total delay of around 20-30 ps (dependent on process), each delayunit can be tuned to within a very high accuracy. This is particularlyimportant for detecting high frequency pulse sequences. The delayelements are preferably non-clocked (asynchronous) delay elements.

The above arrangement can provide very high accuracy delay tuning.However, for long delays the number of delay elements would becomeimpractical if using the shortest delay elements possible. Therefore thetunable delay elements preferably use a two stage coarse/fine delaytuning arrangement. Each delay unit preferably further comprises aplurality of second delay elements connected in series to the output ofthe first multiplexer and a plurality of second delay element tapsbetween the second delay elements, wherein the second delay element tapsare inputs to a second multiplexer and wherein the output of the secondmultiplexer is selected from among the inputs according to a second tapselect input. The second delay elements preferably provide shorterdelays than the first delay elements. The first delay elements can bearranged with longer durations so that fewer elements are required tomake up the desired longer delay. The number of these longer delayelements can still be varied to provide coarse tuning. Fine tuning canthen be provided by means of the second, shorter delay elements whichadd extra delay onto the coarse setting in fine increments until thedesired delay setting is achieved.

With the above tuning arrangements, the delay units of the delay linemay be calibrated in a number of different ways. In some preferredembodiments a reference delay unit is provided for calibrating the delayunits of the delay line.

The reference delay unit may simply be a circuit element with awell-known and/or predetermined delay time which can be used forcomparison with the tunable element which is undergoing tuning. Howeverpreferably the reference delay unit comprises a plurality of referencedelay elements and a plurality of reference delay element taps betweenthe reference delay elements, wherein the reference delay element tapsare inputs to a reference multiplexer and wherein the output of thereference multiplexer is selected from among the inputs according to areference tap select. With this arrangement the reference delay elementcan provide a variable output, i.e. a number of different referencedelays, according to the given reference tap select. This arrangement isuseful when adjusting the detector for receiving a different chip rate.Preferably the detector further comprises a comparator arranged tocompare the output of the reference delay unit with the output of one ormore of the delay line delay units.

The above tuning/calibration scheme allows for absolute calibration ofthe delay units. However this is not always required. In othersituations it is sufficient to use relative tuning, i.e. tuning thedelay line relative to itself, e.g. simply to ensure that all delay linetaps are equally spaced. Relative tuning can of course be used togetherwith absolute tuning, e.g. by tuning one element absolutely and thentuning the remaining elements relative to the tuned element. Preferablytherefore the detector further comprises a comparator arranged tocompare the output of one delay line delay unit with at least one otherdelay line delay unit.

According to a further aspect, the invention provides a method ofdetecting a pulse sequence in an incoming signal in continuous time,comprising: quantizing the incoming signal into discrete levels;generating a plurality of delayed versions of the incoming signal;comparing each of said delayed signals with a stored correlation value;counting the results of the comparison in a continuous time counter; andcomparing the counter output with a threshold value.

The preferred features described above in relation to the apparatus alsoapply equally to the method.

Preferably therefore the step of counting comprises: taking a pluralityof counter inputs; providing a plurality of adding stages eachcomprising a stack of one or more outputs, each counter input providingan input to an adding stage, wherein the adding stages are arrangedsuccessively from a first adding stage to a last adding stage; settingthe single output of the first adding stage to an output value; and foreach succeeding adding stage, taking the outputs from the immediatelypreceding stage and if the appropriate counter input represents a count,adding a new output at the bottom of the stack and if the counter inputrepresents no count, adding a new output at the top of the stack.

Preferably the first adding stage takes its input from the first counterinput and if the first counter input represents a count, the firstadding stage is arranged to set its single output to represent a countand wherein if the first counter input represents no count, the firstadding stage is arranged to set its single output to represent no count,and in each succeeding adding stage, if the counter input represents acount, the adding stage is arranged to add a new output at the bottom ofthe stack representing a count, and if the counter input represents nocount, the adding stage is arranged to add a new output at the top ofthe stack representing no count.

Each adding stage may comprise one or more switching elements connectingthe inputs to the outputs. The switching elements may be multiplexerseach connecting one of two neighbouring inputs to an output. Theswitching elements may be demultiplexers each connecting an input to oneof two neighbouring outputs. The counter may apply a delay to each ofits inputs to compensate for the propagation delay through the addingstages.

The method may further comprise tuning the delays applied to thereceived signal by passing the received signal through tunable delayunits. Each delay unit may comprise a plurality of first delay elementsand a plurality of first delay element taps between the first delayelements, and the method may further comprise tuning at least one delayunit by selecting one of the first delay element taps as the output ofthe delay unit.

Each delay unit may further comprise a plurality of second delayelements connected to the selected output of the plurality of firstdelay elements and a plurality of second delay element taps between thesecond delay elements, and the step of tuning may comprise selecting oneof the second delay element taps as the output of the delay unit.Preferably the second delay elements provide shorter delays than thefirst delay elements.

The method may further comprise calibrating at least one delay unitsagainst a reference delay unit. The reference delay unit may comprise aplurality of reference delay elements and a plurality of reference delayelement taps between the reference delay elements, and tuning the delayunit against the reference delay unit may comprise selecting a referencedelay element tap and tuning the delay unit to match the delay given bythe selected reference delay element tap.

The method may further comprise comparing the output of the referencedelay unit with the output of one or more of the delay units.

The step of tuning may comprise tuning the output of one delay unit tothat of at least one other delay unit.

The tunable delay line described above is believed to be independentlyinventive. Therefore according to a further aspect, the inventionprovides a tunable delay line comprising one or more delay units and oneor more delay line taps; wherein each delay unit comprises a pluralityof first delay elements and a plurality of first delay element tapsbetween the first delay elements, wherein the first delay element tapsare inputs to a first multiplexer and wherein the output of the firstmultiplexer is selected from among the inputs according to a first tapselect input.

According to another aspect, the invention provides a method of tuning atunable delay line, the delay line comprising one or more delay unitsand one or more delay line taps and each delay unit comprising a firstplurality of delay elements and a first plurality of delay element tapsbetween the first delay elements, the method comprising selecting one ofthe first delay element taps as the output of the delay unit.

The preferred features of the tunable delay line described above inrelation to the symbol detector and method apply equally to the tunabledelay line on its own.

The above described thermometer coded counter is also believed to beindependently inventive. Therefore according to a further aspect, theinvention provides a continuous time counter comprising: a plurality ofcounter inputs; and a plurality of adding stages, each adding stagecomprising a stack of one or more outputs, and each counter inputproviding an input to an adding stage, wherein the adding stages arearranged successively from a first adding stage to a last adding stage,wherein the first adding stage is arranged to set its single output to afirst output value, and wherein each succeeding adding stage is arrangedto take the outputs from the immediately preceding stage and if theappropriate counter input represents a count, the adding stage isarranged to add a new output at the bottom of the stack and if thecounter input represents no count, the adding stage is arranged to add anew output at the top of the stack.

According to another aspect, the invention provides a method ofcontinuous time counting comprising: taking a plurality of counterinputs; providing a plurality of adding stages each comprising a stackof one or more outputs, each counter input providing an input to anadding stage, wherein the adding stages are arranged successively from afirst adding stage to a last adding stage; setting the single output ofthe first adding stage to an output value; and for each succeedingadding stage, taking the outputs from the immediately preceding stageand if the appropriate counter input represents a count, adding a newoutput at the bottom of the stack and if the counter input represents nocount, adding a new output at the top of the stack.

Preferably the first adding stage takes its input from the first counterinput and if the first counter input represents a count, the firstadding stage sets its single output to represent a count and wherein ifthe first counter input represents no count, the first adding stage setsits single output to represent no count, and in each succeeding addingstage, if the counter input represents a count, the adding stage adds anew output at the bottom of the stack representing a count, and if thecounter input represents no count, the adding stage adds a new output atthe top of the stack representing no count.

The preferred features of the counter described above in relation to thesymbol detector and method apply equally well to the counter taken onits own.

Preferred embodiments of the invention will now be described by way ofexample only and with reference to the accompanying drawings in which:

FIG. 1 schematically shows a continuous time binary value (CTBV)cross-correlator according to a first embodiment of the invention;

FIGS. 2A and 2B schematically show CTBV counters according toembodiments of the invention;

FIGS. 2C and 2D illustrate the values in the adding stages of thecounters of FIGS. 2A and 2B respectively;

FIG. 3 shows a high-order Gaussian pulse and a corresponding detectoroutput;

FIG. 4 shows a programmable delay element according to embodiments ofthe invention;

FIG. 5 shows an arrangement for setting the delay adjustment of thedelay element of FIG. 4;

FIG. 6 illustrates delay matching against a clock signal;

FIG. 7 illustrates delay matching against a reference delay unit; and

FIG. 8 shows a programmable delay element according to embodiments ofthe invention with coarse and fine tuning.

As shown in FIG. 1 the Continuous Time Binary Value (CTBV)cross-correlator 100 is based on a single bit or binary quantization ofthe incoming signal. The quantizer 110 may be a comparator basedsolution or it may be a zero-level quantizer giving the sign of theincoming signal (i.e. sign(S)). It should be noted that, although theembodiments described herein are in relation to binary quantization, thecircuit can easily be adapted for a multi-level system, e.g. a 3-level(ternary) quantizer and comparators.

A pulse extender 120 is shown in FIG. 1, although as indicated this isan optional feature. The pulse extender 120 can facilitate processing aseach detected pulse may be extended to a suitable width depending on thetransmission center frequency or the period of the sequence. As thequantizer 110 is not clock-based, the pulse length is simply the timethat the incoming signal spends above the quantizer threshold. In someinstances this could be too short for reliable comparisons to be madelater in the circuit so the pulse extender 120 is used to ensure aminimum pulse length.

For cross-correlation template matching (e.g. in a symbol detector) theincoming sequence of bits is stored in a delay-line 130 made up of delayunits 140 with suitable delays matching the expected period of thetransmitted pattern (i.e. the expected chip rate). By tapping the delayline at equally spaced points 150 corresponding to the expected period,it is possible to continuously process the incoming sequence of bitslooking for the desired pattern. The delay units 140 are alsoasynchronous units (not clock-based).

The unit delays 140 between taps 150 make up a discrete, temporal(time-domain) pattern of the incoming signal suitable forcross-correlation with a binary template 160. Importantly, it can beobserved that the delay line may give dense samples. For example, if adouble inverter structure is used for the unit delay 140, it is possibleto achieve delays of the order of only 20 ps (equivalent to a samplingrate of 50 GHz). Quite long unit delays 140 may also be constructed ofthe order of tens of nanoseconds, but due to production variation,calibration may be required. These long-delay correlators areparticularly suitable for power efficient symbol detection, e.g. inimpulse radio communication. Shorter delay correlators are particularlywell suited to high frequency (e.g. Ultra Wide Band) pulse detection.

FIG. 1 shows a template register 160 in which the bit sequence of anexpected symbol may be stored. It will be appreciated that in otherembodiments this register may be static or time variable, e.g. it mayrepresent a time dependent bitstream from another signal forcross-correlation or from the same signal for auto-correlation. Forexample, it may represent a time-delayed version of the input signal.

As indicated above, in this embodiment the template register 160 issimply a binary sequence. Depending on the coding strategy used, not alltemplate bits may be part of the code. For example, if the codingstrategy involves pseudo-noise patterns with sparse filling, the logicaloperation 170 (OP in FIG. 1) could be an AND operation. This arrangementlooks for matches of positive, ‘1’ values but ignores the correlation of‘0’ values. Alternatively, for codes with all chips filled (likebiphasic codes), the detector might match for both ‘0’ and ‘1’ and coulduse XOR as the operation 170. In general the required combinatoriallogic function greatly depends on the coding scheme being used.

The outputs of the operation 170 are fed into continuous time counter180 which outputs a thermometer coded output 190. The thermometer codedoutput 190 is passed to a comparator 195 which compares the counteroutput to a threshold value. It will be appreciated that the counter 180may generate its output in a form other than a thermometer code, e.g. itmay be a floating value output as described elsewhere, or it may be anyother appropriate format.

A CTBV pulse counter 200 may be designed as shown in FIG. 2. Thecross-correlation result is a set of bits with ‘1’ indicatingcorrelation match. The number of ‘1’s appearing at the OP 170 outputsgives the instantaneous cross-correlation between the incoming bitstreamand the template 160.

By controlling the switches 205 with the OP outputs 170, the counter can“steer” ‘0’s and ‘1’s, thus giving a continuous measure (count) of thetemplate match as a stack of ‘1’s on the output. This output coding isknown as thermometer coding with higher “temperature” (i.e. greaterstack of ‘1’s) indicating better match. Simple combinatorial logic maybe used to determine the matching level, e.g. by comparing to athreshold level 210. The threshold may be an array of values with a ‘1’placed at the appropriate height (array location) for the acceptablematch level, e.g. if at least three bits in the input stream must bematched then a ‘1’ is placed in the third array element (see FIGS. 2Cand 2D). It will be appreciated that this arrangement is highlyimplementation dependent. In this way, the symbol detector can detectsymbols probabalistically, e.g. when a threshold number of matches havebeen made (this need not correspond to a perfect match). In oneembodiment, the switches 205 are made as multiplexers in MOS technology.Other forms of switch 205 may also be used and will depend on thepurpose for which the counter is being used. For example transistors(bipolar or FETs) could be used. Equally, physical switches such asrelays could be used in large scale applications. For larger systems(e.g. with a large number of counter stages), and especially in highfrequency applications, the propagation delay through the switchingnetwork must be compensated for. This can be done in a number of ways.One way is to introduce delay elements into the path of the counterinputs, e.g. a series of inverters. Alternatively, the comparators usedto compare the incoming signal with the template elements (correlationvalues) can be slowed down or speeded up by varying the strengths of thetransistors used therein. Another way of compensating for thepropagation delay is to use one or more pulse extenders 120 to extendthe incoming chips so that they are long enough to accommodate thelongest propagation delay through the counter 200. In other words theswitches 205 in the last counter input must remain in their correctpositions long enough for the signal from the first counter input (andall intervening inputs) to propagate through all counter stages andreach the output. Alternatively, the compensation delay may be builtinto the switches of the counter, these elements being designed to havean appropriate delay according to their location within the circuit.Combinations of these techniques may be used to compensate for thepropagation delay.

To describe the operation of the counter 200 in more detail, the counter200 is made up of a number of stages 220 a-d, one for each of the OP 170outputs (i.e. one for each template bit). The first stage 220 a simplyintroduces a ‘1’ or a ‘0’ according to the result of the firstcomparator 170 a. Each subsequent stage 220 b-d adds the result of itscomparator 170 b-d to the result of the previous stage. In order topreserve the thermometer coding within each stage, if the stage isadding a ‘1’ bit (e.g. for a successful comparison), this is added atthe bottom of the stack and if the stage is adding a ‘0’ bit (e.g. foran unsuccessful comparison), this is added at the top of the stack. Allelements of the previous stage's stack maintain their order. In thisway, all the ‘1’s are inserted into the bottom of the stack and arethereby kept together. All remaining positions in the stack are made upof ‘0’s inserted from the top. The final stage output 220 d is the fullthermometer coded count and becomes the output of the counter 200. Thenumber of ‘1’s in the stack (and therefore the height of the ‘1’s in thethermometer) represents the count of the number of ‘1’s in the inputs.In the cross-correlator of FIG. 1, this represents the extent of thecorrelation between the incoming signal and the template register.

This operation is illustrated in FIG. 2C. At the top of the Figure, thequantized values of the incoming signal are shown along with bit valuesof a stored template and the results of the comparator which combinesthe signal and template values (in this case using an AND operation).Underneath these values and in line therewith, the adding stages areshown. At the left hand side, the first adding stage is set with asingle ‘1’ value as the comparator output is ‘1’. In the second stage(one stage to the right), the second comparator result has been added inat the bottom of the stack. In the third stage (one stage further to theright) a third stack value of ‘0’ is added at the top of the stack. Theprocess repeats until the eighth stage shows a stack of four ‘1’s on thebottom and four ‘0’s on the top. These values correspond to the four‘1’s and four ‘0’s shown in the comparator output at the top, butrearranged into a thermometer code.

Finally, the dashed box at the end of FIG. 2C represents a thresholdvalue. A ‘1’ has been set in the fourth box of the array to indicatethat a symbol will be detected upon counting four successful matches. Ascan be seen, the template for the expected signal actually contains five‘1’s and the incoming signal did not match one of those ‘1’s, but thematch of four ‘1’s is considered sufficient to indicate that theincoming signal was intended to contain the correct symbol pattern.

An alternative arrangement of the counter is illustrated in FIG. 2B. Inthis figure, a ‘1’ is inserted at the left hand end, regardless of thecounter inputs. The adding stages then add ‘0’s either on above or belowthe ‘1’ according to the counter inputs, thus causing the ‘1’ to floatat a certain height within the adding stage. The height at which itfloats is determined by the number of successful matches, i.e. thenumber of ‘0’s added into the bottom of the stack. FIG. 2D illustratesthe values within the adding stages in corresponding fashion to that ofFIG. 2C. It will be seen that FIG. 2D contains nine vertical arrays asthe first array simply injects the first ‘1’ and is not dependent on acounter input.

Two particular applications of the CTBV counter 200 will now bedescribed, although it will be appreciated that the counter is notlimited to these applications.

Example Application 1—Higher-Order Gaussian Pulse Detector

One particularly challenging task which is solved by the CTBV counter200 is pulse discrimination for impulse radio. Although different pulseshapes may be used, a popular pulse shape is an approximation to aGaussian shaped pulse or Gaussian-derivative shaped pulse. Gaussian andGaussian-derivative pulses have good spectral filling properties and aretherefore useful in the unlicensed UWB bands. In the USA the FCC permitsunlicensed transmission at powers below −41.3 dBm/MHz between 3.1 and10.6 GHz. In Europe, the band is narrower at 6-8.5 GHz. As regulations(especially outside the USA) are limiting the unlicensed UWB bands,higher order Gaussians (higher order derivatives) are required in orderto fill the permitted spectrum.

FIG. 3 shows a typical Gaussian-derivative pulse shape 300 showntogether with the quantized output 310 using two different thresholdvoltage levels. By using two CTBV counters 200, both the positive 310 aand negative 310 b pulses may be counted. The unit-delay of the CTBVcounter is adapted to the center frequency of the transmitted signal. Bydetecting and counting the received pulses, the detector can readilydiscriminate between received pulses and background noise. As describedabove, the detection results of two counters (for counting both positiveand negative peaks) may be combined for improved time-domain pulsediscrimination. On the other hand, a single counter (for counting justpositive or negative peaks) may be used if that provides adequatediscrimination. By adapting the threshold voltages of the quantizer andsetting the number of detectable peaks in the template register 160,good pulse discrimination can be achieved. The continuous time,non-clocked processing of the counter 200 allows pulse detection to beperformed at high frequency and with good power efficiency.

Example Application 2—CTBV Symbol Detector

The second example application of the CTBV counter 200 is for symboldetection in pulsed radio communication (e.g. in impulse radar or inwireless communication systems). The unit-delay of the CTBV-counter 200is tuned to the chip length (i.e. the time duration of one chip) of thePN-coded symbol (which is often of the order of tens of nanoseconds). Bysetting the appropriate bit-sequence (i.e. the sequence of chips whichform the symbol) in the template register 160, a runningcross-correlation is computed. By using appropriate detection logic onthe counter output 220 d, symbol detection is feasible. It should benoted that no clock-synchronization is required between transmitter andreceiver because all processing is carried out in continuous time.However, relative chip-clock accuracy in the transmitter is importantsince the delay line taps 150 are calibrated to the transmitted chipclock frequency. Deviations between the transmitter chip clock and thedelay line taps 150 may lead to degraded symbol detection, e.g. symbolsbeing missed. Some leeway is given by using the pulse extender 120 shownin FIG. 1. By extending the pulses, small timing inaccuracies can beaccommodated while still ensuring that sufficient temporal overlapoccurs within the counter for an accurate correlation to be produced. Itshould be noted that clock inaccuracy is also important within theduration of one symbol. With current crystal quality, few problemsshould occur for short symbols, but care needs to be taken for longersymbols.

For longer counting operations (a large number of inputs), the inputsmay be grouped together and the groups passed to separate counters 200for parallel counting before combining the results. Alternatively, theadding stages 220 a-d of the counter 200 may be grouped together and theinterim counter results can be latched or stored for use in thesubsequent adding stages 220 a-d while the previous adding stages 220a-d can be reused for the next counting operation. This arrangementallows pipelining of the counter.

The significant variations in production spread of modern technologydirectly map to variations in gate delays. In the CTBV solutionsdescribed herein, clocking is avoided. Instead, these solutions exploreinherent gate delays in delay lines. It is therefore important toaccount for these major variations in gate delays. The inventiontherefore provides programmable delay units which can be adjusted toprovide a varying delay and can therefore be accurately calibrated to aselected desired delay.

In FIG. 4 a programmable delay unit 400 is shown. The delay unit 400 isitself formed from a delay line 410 of shorter delay elements 420. Delayline taps 430 are taken from between the shorter delay elements 420 andthese taps 430 form the inputs to a multiplexer 440. The multiplexer 440is controlled by a tap select input 450 which selects which one of themultiplexer inputs 430 is to be passed to the multiplexer output 460. Inthis way, by varying the tap select input 450 to the multiplexer 440,the number of shorter delay elements 420 in the delay unit 400 can bevaried, thus varying the overall delay of the delay unit 400.

The simplest and smallest delay element can be formed from two digitalinverters connected in series. In a typical nanometer process, twocascaded inverters will provide a minimum time delay, τ_(min) of theorder of 20-30 ps, although this figure is somewhat dependant on thequality of the layout. Although there is some spread in this minimumdelay, the precision is still sufficient for calibration of longerdelays such as 1 ns or longer.

Calibration can be performed in different ways. It is convenient todistinguish between absolute calibration requiring an external timingreference to which the delay unit 400 is to be matched and relativecalibration requiring matching of two on-chip delays (i.e. matching oneon-chip delay unit to another on-chip delay unit). This relativecalibration may be done by matching delay line delay units to a separatepre-calibrated on-chip reference delay unit or it may involveself-tuning of the delay line by tuning the delay line delay units toeach other in order to provide a consistent time separation of the taps.

A delay unit 400 may be calibrated to an external clock signal 510 byusing an edge triggered D flip flop 520. The non-delayed clock drivesthe flip flop 520 (via the enable input) to sample the delayed output460 from the multiplexer 440. Using a positive edge triggered D flipflop 520, if the flip flop output 530 is high this indicates that theoutput 460 of the multiplexer 440 went high earlier than the clock 510and the tap select 450 should be changed to increase the delay until theflip flop output 530 goes low. On the other hand, if the flip flopoutput 530 is low this indicates that the output 460 of the multiplexer440 went high later than the clock 510 and the tap select 450 should bechanged to decrease the delay until the flip flop output 530 goes high.An iterative search for the clock edge can be performed.

This arrangement is shown in FIG. 5 and the adjustment procedure isillustrated in FIG. 6 which shows the clock signal 610 against theadjustable delay signal 620 for various delays (i.e. for various tapselects).

If a good quality clock is used, both the negative (half-period) andpositive (full-period) of the multiplexer output 460 may be sampled.This can be done either by introducing logic to select whether therising edge or falling edge is sampled or two separate flip flops can beemployed one for sampling on the rising edge and one on the fallingedge. Sharp clock transitions (good quality clock) reduce clock jitterand sampling on both rising and falling edges allows double the numberof calibration measures, thereby providing more accurate calibration.Also, by sampling half periods, a higher frequency calibration can beperformed without introducing a higher frequency clock.

Each delay unit of a delay line (e.g. the delay line 130 of FIG. 1) canbe tuned or calibrated in this way, with a tap select 450 beingidentified for each delay unit so as to create a tap-select pattern forthe delay line as a whole. The tap-select pattern after this calibrationprocedure achieves an absolute matching to within the resolution of theminimum delay time, τ_(min), i.e. of the order of 20-30 ps for a sub 100nm process.

A relative calibration may also be done using a D flip-flop. Anappropriate set up is shown in FIG. 7. In this procedure, a calibrateddelay unit (Reference delay unit 710) is used in place of the clockreference. The calibrated delay unit 710 may be either an on-chippre-calibrated delay unit (e.g. calibrated in the factory) or it may beanother delay unit in the delay line of the main detector circuit. Toconduct the calibration, both the reference delay unit 710 and the delayunit to be calibrated (tunable delay unit 720) are triggeredsimultaneously by trigger input 730 and the outputs of the two delayunits 710, 720 are used as the inputs to the D flip flop 740 (the inputsmay be either way round). As in the previous example, the output 750 ofthe D flip flop 740 indicates the relative time order of the two inputs(i.e. which triggers first) and the tap-select 760 of the delay unitunder calibration 720 can be adjusted accordingly until a match isobtained, thus ensuring as close as possible a match between the twodelays. The tap select 770 of the reference delay unit 720 can be variedto select different reference timings.

By adding in suitable multiplexers, any number of delay units may becalibrated to the single calibrated delay unit 720 or to each other.

As can be seen, these tunable delay lines can be extremely useful inCTBV-style circuits. Two further applications of tunable delay lineswill now be described, although these are not limiting on the scope ofthe invention. It will be appreciated that tunable delay lines find usesin numerous circuits.

Example Application 3—Tunable Ranging for Impulse Radar

In impulse (gated) radar systems, a tunable delay is required for theranging function. The radar system emits pulses and looks for echoes ofthose pulses reflected from objects within a certain range, i.e. withina certain time of the transmission. Because of the severe transmissionpower restrictions of the UWB spectrum, an individual reflected pulsecan in some circumstances be hard to detect, so instead a large numberof received pulses are generally averaged before the signal can bedistinguished above the noise level.

The radar unit selects a particular sampling time in the receivedsignal, a certain time after pulse transmission and corresponding to aparticular target distance. The system repeatedly samples and averagesthe signal at that point in time to determine if pulses are beingreflected at that target distance. The target distance being sampled(i.e. the point in time in the received signal) is determined by a delayunit placed between the transmitter and the sampler. In other words, thetransmitted pulse (or the trigger used for pulse generation) issimultaneously passed through the delay unit and used as the trigger tostart the received signal sampler. By varying the length of this delay,the sample is controlled to sample different points in time andtherefore different distances.

By accurately programming the time between when the outgoing impulsesare transmitted and the backscattered or reflected signal is sampled,the radar range can be set accurately. By varying the delay, objects canbe detected at different distances. Electromagnetic waves travel at aspeed close to light, thereby demanding highly accurate time adjustmentsin order to get accurate distance measurements. In a single sample radarsystem (i.e. one which only looks at a single target distance at a giventime), the depth resolution is dependent on the fine-tuning of theranging delay. For example, if the ranging delay is only known to withina few manometers, then the distance is only known to within about ametre. For short-range radar systems, resolution requirements are reallychallenging. In long-range radar system, a distance accuracy to within afew metres may be considered highly accurate, but over short ranges,such as a few metres, a higher precision (i.e. cm or mm precision)becomes desirable. With the system described here, assuming τ_(min)=20ps, ranging may be tuned to about 3 mm precision.

A programmable and calibrated delay element for quite accurate rangingmight be implemented efficiently by combining coarse, tunable delayswith ρ_(min) delays. FIG. 8 shows an example of a tunable coarse delayline 810 with unit delays 820 of the order of 2 ns. These coarse unitdelays 820 may be implemented with slow circuits for area efficiency. Asin the previous examples, the overall delay can be tuned by selecting avarying number of these 2 ns delay elements using first multiplexer 830.

Unfortunately slow transitions increase production spread and make iteven more difficult to design accurate delays. However, if all τ_(2ns)delays are intentionally made a little too short, then a programmablenumber of τ_(min) delays may be added to make up the shortfall to withinan accuracy of τ_(min).

For calibration of this delay line, an accurate clock 840 matching theτ_(2ns) delay (i.e. 500 MHz) is used as a trigger, and the abovecalibration scheme using a D flip-flop 850 is applied. Calibration maybe done against any integral number of clock periods. To calibrate anentire delay line, each coarse delay line tap is calibrated in turn. Forexample, first the coarse tap select 860 can be set to the first tap andthe fine tap select 870 is then varied until the total combined delayoutput from second multiplexer 880 matches the 2 ns clock cycle. Thefine tap select 870 is stored with the first coarse tap select 860. Nextthe coarse tap select 860 is set to the second tap and the fine tapselect 870 is again varied until the total combined delay matches thetwo 2 ns clock cycles (i.e. total delay of 4 ns). This new fine tapselect 870 is stored with the second coarse tap select 860. Thisprocedure is continued for the whole delay line. After calibration eachcoarse delay tap setting is matched by a fine tune setting giving acalibrated result with τ_(min) resolution. Assuming τ_(min)=20 ps, the 2ns coarse tune delay line has been calibrated to within 1%. In otherwords, this arrangement provides a tunable delay line with tap selectsseparated by 2 ns+/−20 ps.

Alternatively, the coarse and fine tap tunable delay line shown in FIG.8 can form a single delay element of a tunable delay line. This providesgreat versatility of accurate timings in a tunable delay line. It willbe appreciated that further levels of refinement may be added, e.g.three or more levels of coarseness may be used.

Example Application 4—Tunable Delay Line Matching Chip Rate in SymbolDetectors

As another example, the tunable unit-delay may be used in CTBV symboldetectors or cross-correlators 100 as shown in FIG. 1.

If it is desired to detect symbols with a chip-duration of 20 ns (i.e.50 MHz chip clock), then the unit delays 140 of the delay line 130 inFIG. 1 must accurately match the 20 ns chip-duration in order to samplethe incoming signal correctly. In fact 20 ns is a long delay comparedwith τ_(min) of say 20 ps and the accumulated deviations along the delayline will be significant. By using a tunable delay unit (e.g. as shownin FIG. 4 or FIG. 8) combined with a periodic signal (clock), a simpleprocedure may be applied for unit-delay adjustment as follows:

The symbol detector 100 of FIG. 1 is set up with the delay line 130comprising tunable delay elements 140. The tunable delay elements 140may be the delay unit 400 of FIG. 4. The detector 100 is also set up touse AND-correlation (i.e. operation 170, OP=AND gate) and a 50 MHz clockis supplied as the input. The counter threshold 195 is set to 2 (i.e. todetect a symbol if two ‘1’s are correctly matched against the template)and for the first unit delay 140, the symbol template 160 is set to“11000000 . . . ”.

The first unit delay 140 is then tuned. When the first delay element 140is close enough to 20 ns, the detector 100 detects both ‘1’s of thesymbol in the received signal stream and the system will find “symboldetected”. For example with the arrangement of FIG. 2A, the counteroutputs a thermometer code with two ‘1’s which matches the threshold 195set previously. By increasing/decreasing the unit-delay 140, the minimumand maximum tuning settings for symbol detection can be determined. Atuning setting around midway between the maximum and minimum settingswill give a unit-delay 140 close to 20 ns and will provide the bestsetting for tuning of the next unit-delay.

The procedure continues by moving a ‘1’ to the right in the symboltemplate, e.g. “10100000 . . . ” and repeating the procedure tocalibrate the next delay element 140, by tuning the next delay element140 until the symbol is detected. In this way all unit-delays are tunedto match the chip-rate clock. It should be noted that the accumulatederrors introduced early in the delay line are accounted for by thissequential tuning procedure.

It will be appreciated that the above procedure is just one example of atuning procedure and different logical operators 170 and symboltemplates 160 could also be used to achieve the same goal.

The above embodiments may readily be combined together or usedseparately. For example, the tunable delay line and thethermometer/floating coded counter (FIGS. 2A and 2B) can each or both beused in the symbol detector or pulse detector 100 described above.Equally, these circuits are not tied to use in the symbol or pulsedetectors and can be used in a large variety of other circuits and otherapplications. The above descriptions are provided merely as examples andthe skilled person will readily selectively combine the various elementsof the different embodiments as desired.

1. A continuous time cross-correlator comprising: a quantizer forquantizing the incoming signal into discrete levels; a delay linecomprising one or more delay units separating a plurality of delay linetaps; for each of said delay line taps, a comparator for comparing thesignal level of the delay line tap with a correlation value; acontinuous time counter for taking the outputs of the plurality ofcomparators as its inputs, counting the results of the comparisons andoutputting the results of the comparisons; and an output comparator forcomparing the counter output with a threshold value.
 2. Thecross-correlator of claim 1, wherein the counter comprises: a pluralityof counter inputs; and a plurality of adding stages, each adding stagecomprising a stack of one or more outputs, and each counter inputproviding an input to an adding stage, wherein the adding stages arearranged successively from a first adding stage to a last adding stage,wherein the first adding stage is arranged to set its single output to afirst output value, and wherein each succeeding adding stage is arrangedto take the outputs from the immediately preceding stage and if theappropriate counter input for the adding stage represents a count, theadding stage is arranged to add a new output at the bottom of the stackand if the counter input represents no count, the adding stage isarranged to add a new output at the top of the stack.
 3. Thecross-correlator of claim 2, wherein the first adding stage takes itsinput from the first counter input and if the first counter inputrepresents a count, the first adding stage is arranged to set its singleoutput to represent a count and wherein if the first counter inputrepresents no count, the first adding stage is arranged to set itssingle output to represent no count, and wherein in each succeedingadding stage, if the counter input represents a count, the adding stageis arranged to add a new output at the bottom of the stack representinga count, and if the counter input represents no count, the adding stageis arranged to add a new output at the top of the stack representing nocount.
 4. The cross-correlator of claim 2, wherein each adding stagecomprises one or more switching elements for connecting the inputs tothe outputs.
 5. The cross-correlator of claim 4, wherein the switchingelements are multiplexers each arranged to connect one of twoneighbouring inputs to an output.
 6. The cross-correlator of claim 4,wherein the switching elements are demultiplexers each arranged toconnect an input to one of two neighbouring outputs.
 7. Thecross-correlator of claim 2, wherein the counter is arranged to apply adelay to each of its inputs to compensate for the propagation delaythrough the adding stages.
 8. The cross-correlator of claim 1, whereinthe delay units of the delay line are tunable.
 9. The cross-correlatorof claim 8, wherein each delay unit comprises a plurality of first delayelements and a plurality of first delay element taps between the firstdelay elements, wherein the first delay element taps are inputs to afirst multiplexer and wherein the output of the first multiplexer isselected from among the inputs according to a first tap select input,the output of the first multiplexer forming the output of the delayunit.
 10. The cross-correlator of claim 9, wherein each delay unitfurther comprises a plurality of second delay elements connected to theoutput of the first multiplexer and a plurality of second delay elementtaps between the second delay elements, wherein the second delay elementtaps are inputs to a second multiplexer and wherein the output of thesecond multiplexer is selected from among the inputs according to asecond tap select input, the output of the second multiplexer formingthe output of the delay unit.
 11. The cross-correlator of claim 10,wherein the second delay elements provide shorter delays than the firstdelay elements.
 12. The cross correlator of claim 8, further comprisinga reference delay unit for calibrating the delay units of the delayline.
 13. The cross-correlator of claim 12, wherein the reference delayunit comprises a plurality of reference delay elements and a pluralityof reference delay element taps between the reference delay elements,wherein the reference delay element taps are inputs to a referencemultiplexer and wherein the output of the reference multiplexer isselected from among the inputs according to a reference tap select. 14.The cross-correlator of claim 12, further comprising a comparatorarranged to compare the output of the reference delay unit with theoutput of one or more of the delay line delay units.
 15. Thecross-correlator of claim 8, further comprising a comparator arranged tocompare the output of one delay line delay unit with at least one otherdelay line delay unit.
 16. A method of detecting a pulse sequence in anincoming signal in continuous time, comprising: quantizing the incomingsignal into discrete levels; generating a plurality of delayed versionsof the incoming signal; comparing each of said delayed signals with acorrelation value; counting the results of the comparison in acontinuous time counter; and comparing the counter output with athreshold value.
 17. The method of claim 16, wherein the step ofcounting comprises: taking a plurality of counter inputs; providing aplurality of adding stages each comprising a stack of one or moreoutputs, each counter input providing an input to an adding stage,wherein the adding stages are arranged successively from a first addingstage to a last adding stage; setting the single output of the firstadding stage to an output value; and for each succeeding adding stage,taking the outputs from the immediately preceding stage and if theappropriate counter input for the adding stage represents a count,adding a new output at the bottom of the stack and if the counter inputrepresents no count, adding a new output at the top of the stack. 18.The method of claim 17, wherein the first adding stage takes its inputfrom the first counter input and if the first counter input represents acount, the first adding stage sets its single output to represent acount and wherein if the first counter input represents no count, thefirst adding stage sets its single output to represent no count, andwherein in each succeeding adding stage, if the counter input representsa count, the adding stage adds a new output at the bottom of the stackrepresenting a count, and if the counter input represents no count, theadding stage adds a new output at the top of the stack representing nocount.
 19. The method of claim 17, wherein each adding stage comprisesone or more switching elements connecting the inputs to the outputs. 20.The method of claim 19, wherein the switching elements are multiplexerseach connecting one of two neighbouring inputs to an output.
 21. Themethod of claim 19, wherein the switching elements are demultiplexerseach connecting an input to one of two neighbouring outputs.
 22. Themethod of claim 17, wherein the counter applies a delay to each of itsinputs to compensate for the propagation delay through the addingstages.
 23. The method of claim 16, further comprising tuning the delaysapplied to the received signal by passing the received signal throughtunable delay units.
 24. The method of claim 23, wherein each delay unitcomprises a plurality of first delay elements and a plurality of firstdelay element taps between the first delay elements, the method furthercomprising tuning at least one delay unit by selecting one of the firstdelay element taps as the output of the delay unit.
 25. The method ofclaim 24, wherein each delay unit further comprises a plurality ofsecond delay elements connected to the selected output of the pluralityof first delay elements and a plurality of second delay element tapsbetween the second delay elements, wherein the step of tuning comprisesselecting one of the second delay element taps as the output of thedelay unit.
 26. The method of claim 25, wherein the second delayelements provide shorter delays than the first delay elements.
 27. Themethod of claim 23, further comprising calibrating at least one delayunits against a reference delay unit.
 28. The method of claim 27,wherein the reference delay unit comprises a plurality of referencedelay elements and a plurality of reference delay element taps betweenthe reference delay elements, and wherein tuning the delay unit againstthe reference delay unit comprises selecting a reference delay elementtap and tuning the delay unit to match the delay given by the selectedreference delay element tap.
 29. The method of claim 27, furthercomprising comparing the output of the reference delay unit with theoutput of one or more of the delay units.
 30. The method of claim 23,wherein the step of tuning comprises tuning the output of one delay unitto that of at least one other delay unit.